1. Field of the Invention
This invention relates to a body fluid-filtering device including, for example, a hemofiltration type artificial kidney for the effective treatment of a patient suffering from renal insufficiency or the like.
2. Description of the Prior Art
In recent years, medical treatment by blood filtration has been used for patients suffering from, for example, renal insufficiency which is difficult to treat by dialysis. Therefore, strong demand is made for the development of a hemofiltration type artificial kidney. For medical treatment by blood filtration, the blood should be filtered and the filtrate should be discharged in a volume of 20 to 25 l/5 hr, and consequently an artificial kidney is required to have a filtration rate of 66-85 ml/min.
The conventional hemofiltration type artificial kidney includes the hollow fiber type provided with a plurality of fibers, and the plate type provided with a plurality of superposed flat filtering membranes. However, both types present difficulties in respect of filtering capacity, size and manufacturing cost.
The filtering capacity of the above-mentioned conventional type of artificial kidney largely depends on, for example, not only the material of a filtering membrane and the blood flow rate, but also the thickness of a blood flow passage extending through superposed filtering membranes. In this case, blood filtration can be effected at a higher rate by reducing the thickness of the blood flow passage. However, the conventional hollow fiber type artificial kidney has the drawbacks that a limitation is imposed on the reduction of the inner diameter of a fiber from the standpoint of manufacture; the inner diameter of the fiber can be reduced only to about 200 microns at most and therefore blood filtration, for example, fails to be effected at a desired rate; the wall of a filtering membrane is sheared at a low rate; the filtration rate decreases with time due to the deposition of, for example, protein on the filtering membrane; and a change with time is likely to occur in the cut-off molecular weight of components of a body fluid.
The conventional plate type artificial kidney has the drawbacks that a spacing between the plate type filtering membranes is defined by a plurality of separately arranged spacers, presenting difficulties in equalizing the thickness of an inter-membrane space (thickness of the passage) at various points, and reducing said thickness of an inter-membrane space. Thus, it is difficult to increase the filtration capacity in the artificial kidney of this type.
The conventional attempt to resolve the above-mentioned difficulties comprises using two body fluid-filtering devices, whether of the hollow fiber type or of the plate type, or broadening the area of a filtering membrane. However, these attempts are accompanied with the drawbacks that the filtering device itself is unavoidably enlarged, resulting in high manufacturing cost.
In this connection, reference is made to British Pat. No. 1,555,389 which discloses an ultrafilter. This ultrafilter is constructed by superposing a plurality of filtering elements, each of which comprises a substrate prepared from, for example, meshed woven fabric and covered with a filtering medium at least on one side, and placing the superposed filtering elements in a chamber. With the above-mentioned ultrafilter, a body fluid is filtered while passing through a plurality of superposed filtering elements by utilizing the pressure difference.
The superposed filtering elements help to reduce the size of a filtering device as a whole. However, the filtering apparatus of the British patent was still accompanied with the drawbacks that it is difficult to reduce the thickness of an inter-membrane space, and proteins contained in blood tend to adhere to the membrane because of a slow shearing rate of blood against the membrane wall, resulting in insufficient filtering capacity, and noticeable variations in the filtration rate. Further, blood tends to leak into the filtrate in this filtering device.